The effect of gas pressure on the melting behavior of compounds (original) (raw)
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The Journal of Chemical Thermodynamics, 2002
The melting behaviour of binary mixed crystalline materials was measured with adiabatic calorimetry. The experimental results (heat capacity against temperature graphs) are explained in terms of a model that is based on the assumption that the inhomogeneity of the materials can be described by a Gaussian distribution function. Although this model, called ULTRACAL, has only one adjustable parameter, it is capable of reproducing the experimental data with a good degree of accuracy, as demonstrated for various samples of (1,4-dichlorobenzene + 1,4-dibromobenzene). ULTRACAL thus provides a link between the adiabatic calorimetry data and the solid-liquid Tx phase diagram. It is shown that the model can be used for the determination of phase diagrams. By using ULTRACAL to assess the heat capacity against temperature graphs of five samples, the solid-liquid Tx phase diagram of (1,3,5-trichlorobenzene + 1,3,5-tribromobenzene) was determined.
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A novel method for measuring the onset of vapor-liquid phase transition applying an isochoric procedure in a high-pressure micro differential scanning calorimeter is introduced for the first time. Isochoric dew-point measurement is used to measure vapor pressures of CO 2 at different boiling temperatures and dew points of a methane/ethane gas mixture at different pressures or temperatures. The isochoric two-phase bubble-point measurement, similar to the isobaric method, is also demonstrated to measure vapor pressures of methanol at different boiling temperatures. All results are in agreement with the literature data. The isochoric method is found to be superior to the widely used isobaric method. It can be used to measure the onset of vapor-liquid phase transition for a wide range of substances and mixtures, including the ones for which the isobaric method is inapplicable, and it eliminates difficulties usually encountered in the isobaric method. The proposed method along with the findings of this study can pave the way for experimental measurements of phase equilibria in more complex systems.
Thermochimica Acta
An experimental procedure is proposed for direct measurement of the heat involved in the vaporization of a solid organic compound above its normal melting temperature. This technique consists on the fusion of a solid aromatic hydrocarbon, which is then vaporized by a sudden decrease of the pressure. The direct register of heat flow as function of time by differential scanning calorimetry allows the quantifying of the enthalpy of vaporization of compounds such as phenanthrene, β-naphthol, pyrene, and anthracene. Enthalpies of vaporization were measured in an isothermal mode over a range of temperatures from 10 to 20K above the melting temperatures of each compound, while enthalpies of fusion were determined from separate experiments performed in a scanning mode. Enthalpies of sublimation are computed from results of fusion and vaporization, and then compared with results from the literature, which currently are obtained by calorimetric or indirect techniques.
High-Pressure Melting Curves and Liquid–Liquid Phase Transition
Advanced Science Letters, 2010
Breaks on the melting lines in the pressure-temperature (P T m) space are often handled as a hallmark of a liquid-liquid phase transition in pure materials. In this paper it is shown that there is no one-to-one agreement between these virtual breaks and liquid-liquid transitions. Four melting curve is analyzed; selenium, phosphorus, carbon and nitrogen. It is shown that a modified form of the Simon-Glatzel equation can describe experimental data, without introducing any break, demonstrating that the shape of the melting line is not an evidence for any further phase transition.
Abstract Using the thermal analyzer STA 449 F3 Jupiter (NETZSCH – Germany), the effect of the conditions of calibration and experiment on the values of the enthalpies of the polymorphous transition and melting of КNO3, as well as the enthalpies of melting and, respectively, the degree of crystallinity of poly(ethylene oxide)(PEO) used a model solutions for DSC analyses were studied. For comparison, similar data were obtained for a number of commercial crystallizing polymers (PP 6631, Hostaflon 1502, etc.). The thermo physical characteristics obtained by several types of DSC curve calibration by enthalpy are discussed: 1) by the enthalpy of melting of standardized metals in corundum crucibles; 2) by the specific heat capacity of sapphire in corundum crucibles and 3) by the specific heat capacity of sapphire in Pt crucibles. The following experimental conditions were varied: heating rate – 5, 10 and 20 deg/min; crucibles – corundum or platinum; gaseous medium – air (static) or nitroge...
The Journal of Supercritical Fluids, 2013
The effect on the melting temperature depression (MTD) of organic substances and ionic liquids caused by different types of pressurizing gases is analyzed. A high pressure gas produces a combined effect between solubility and pressure that causes the melting temperature to decrease. The authors have previously used phase equilibrium relations to correlate MTD of organic substances and ionic liquids under high pressure carbon dioxide, but other gases were not considered. The Peng-Robinson equation of state with the Wong-Sandler mixing rules showed to be appropriate for correlating the phase equilibrium in these high pressurized systems and is the model used is in this work. Three organic substances (naphthalene, biphenyl and octacosane) under high pressure produced by three gases (ethane, ethylene and carbon dioxide) for which experimental data on MTD are available were considered in this study. Then extension to an ionic liquid under high pressure carbon dioxide and high pressure ethylene was done. The proposed thermodynamic method and the model used show to have the necessary flexibility to acceptably correlate the MTD produced in these systems.
Journal of Advanced Thermal Science Research, 2019
Recently, phase change materials (PCMs) have received significant attention due to their potential for highdensity thermal energy storage. While high-temperature PCMs have received the most focus in the thermal energy storage community, there are potential uses for PCMs with phase transition temperatures close to typical ambient temperatures (15-35°C). For a PCM to be widely used in a large-scale thermal energy storage system, it must meet the cost, safety, and energy density criteria in addition to having an appropriate phase change temperature. Inorganic, hydrated salt PCMs are the most promising, low-temperature PCMs, which can meet all of these criteria. After completing a review of known inorganic PCMs with phase change temperatures in the desired range, six of the more promising PCMs were tested by differential scanning calorimetry (DSC) to determine both their phase change temperatures (Tm) and latent heats of fusion (Hf). The first of these PCMs (potassium fluoride tetrahydrate) was eliminated after successful DSC testing as it became apparent that this PCM had serious health and safety concerns. Two new calcium chloride hexahydrate (CaCl2•6H2O)-based PCMs were also tested: CaCl2•6H2O + potassium nitrate (KNO3) and CaCl2•6H2O + magnesium chloride hexahydrate (MgCl2•6H2O). For CaCl2•6H2O + KNO3, it was found that the melt temperature of the PCM could be varied by changing the percentage of KNO3. In the case of the CaCl2•6H2O + MgCl2•6H2O, phase diagram modeling and physical experiments were used to determine the correct eutectic mixture, which leads to congruent melting/freezing of this PCM. CaCl2•6H2O was also tested by DSC, with found Tm and Hf results similar to those presented in the literature. Finally, sodium sulfate decahydrate (Na2SO4•10H2O) and Na2SO4•10H2O + 25 wt% H2O were tested by DSC. For both of these PCMs, significant phase separation was observed, which must be addressed if these PCMs are to be used commercially.
Thermochimica Acta, 2019
The equilibrium enthalpy of melting ΔH m 0 [J•g −1 ] is an extrapolated thermodynamic quantity attributed to crystallizable macromolecules and widely used to characterize polymers in their semi-crystalline state, for it allows estimating the degree of crystallinity by direct comparison with the enthalpy of melting obtained from differential scanning calorimetry. ΔH m 0 is typically obtained by cross-comparing the results obtained by at least two techniques. This work proposes a simplified experimental protocol to determine ΔH m 0 by the use of Fast Scanning Calorimetry (FSC). This approach applies to any crystallizable polymer for which a specific microstructure can be obtained (i.e. a two-phase semi-crystalline microstructure with a negligible amount of rigid amorphous fraction) and that can also be quenched to its fully amorphous state. Such a two-phase microstructure can be obtained on nanoscale samples through an annealing process performed in situ on the FSC sensor at crystallization temperatures as close as possible to the melting temperature. The enthalpy of melting is then evaluated from the two-phase model for different crystallization times (i.e. different crystallinities) and the ΔH m 0 is obtained by extrapolating the data to the 100% crystalline state. This procedure was applied on samples whose ΔH m 0 values are already available in the literature, but also on more recent biobased polyesters whose thermal properties are still under investigations.